1. Field of the Invention
The present invention relates to an optical disc in which recording/reproducing of information is carried out through irradiation of a light beam, in particular, relates to an optical disc in which recording/reproducing is carried out both on a groove and on a land.
2. Description of the Related Art
An optical disc, which records/reproduces information through an irradiation of a light beam on a record carrier, possesses concurrently a large capacity, high speed accessibility, medium portability, and is utilized in various recording systems such as an electronic calculator file, an audio/visual file, a document file and the like. Further, due to a recent increase of dynamic image, a strong demand for high definition image and the like, demand for an enhancement of recording density is strong.
For a record-enable optical disc, on a disc substrate, a groove for tracking guide of a light beam, a so-called groove, is disposed in advance by an injection molding and the like. In a conventional record-enable optical disc, only on a recording layer on this groove or on a recording layer on a land which corresponds to a space between grooves, a data-mark is recorded. The reason for this is that, since a width of divergence of a light spot (the width where the strength decreases to e.sup.-2 of the central strength) is larger than a groove width or a land width, when recording is carried out on both a groove area and a land area, a cross-talk becomes extremely large, thereby, signal quality is damaged.
On the other hand, it is made clear that, when a depth of a groove is adjusted considering a phase of the reflected light from a recording layer, due to its optical interference effect, the signal from the land portion can be made negligible small during reproduction of the groove portion, in addition, the signal from the groove portion can be made negligible small during reproduction of the land portion. Thereby, with a sufficiently small cross-talk amount, on both the land portion and the groove portion, recording is made possible. Such a recording method is called a land/groove (L/G) recording, and is proposed as one technology enhancing a surface recording density of an optical disc.
However, in the conventional L/G recording, because of lack of consideration on a coverage of a film onto a groove-step, there has occurred a problem that an effective land width and groove width at the position of the recording layer do not coincide. Thereby, recording/reproducing characteristics on the land and the recording/reproducing characteristics on the groove do not coincide, resulting in a problem of damaging an operation margin.
Such a problem is not remarkable in the case of an optical disc in which a recording layer is directly formed on a substrate, for instance, of a WORM disc, however, becomes extremely remarkable in the case of a write-enable optical disc in which a transparent film providing an optical interference function and a protection function is disposed between the substrate and the recording layer.
Further, in order to suppress cross-erase, a so-called deep groove in which a depth of a groove is made deep is proposed, but, in such a deep groove, an influence of the coverage of a film onto the a groove-step becomes more remarkable.
Thus, in the conventional optical disc, it has been very difficult to realize a L/G recording which is low both in cross-talk and cross-erase, furthermore to make coincide the land characteristics and the groove characteristics.
On the other hand, in an optical disc in which a metallic reflective film high in its thermal conductivity is disposed at an upper portion of a surface of an opposite side to the substrate of the recording layer, for instance, such as a magneto-optical disc and a phase-change type optical disc, due to thermal unsymmetry of the land portion and the groove portion, there has occurred a problem that thermal response of the land portion and the groove portion are different. This causes deterioration of recording power margin.
As a simple method making coincide characteristics of the land portion and the groove portion as described above, there is a method in which, by detecting whether a light spot is irradiating the land portion or is irradiating the groove portion, according to the optimum operation condition for the land portion and the groove portion which are written in advance in a lead-in area and the like, recording/reproducing is sorted.
However, when there occurs an error in detection of the land portion and the groove portion in this method, since the unsymmetry of the land portion and the groove portion becomes more remarkable, prevention of the detection error becomes important. Therefore, there is a problem that a circuit scale for preventing the detection error becomes extremely large.